The converter transient response is mainly limited by filter inductance, dv/dt and delay time of the error amplifier (EA) compensation network. High switching frequency operation helps to reduce the filter inductance, but it deteriorates the converter efficiency. Design of the compensation network to minimize the delay time of the compensation network is very important to the transient response under certain switching frequency. The longer delay time the compensation network has, the worse transient response or voltage spikes occur.
Adequate gain and phase margins are required in the conventional linear control for the converter stability. In transient periods, in ideal case, the error amplifier output (EAO) increases in step up load and decrease in step down immediately following the transient load, so there are no voltage spikes in the output voltage. Actually, the EAO is determined by error amplifier performance and the design of the compensation network, if it cannot change as quickly as the load, and then the output voltage spikes occur. Smaller gain and phase margins contribute to smaller delay time and faster transient response, at the sacrifices of converter stability. Thus, there is a trade-off between the transient response and converter stability when designing the converter. Usually high performance and costly error amplifiers such as high product of gain and bandwidth, high dv/dt, is used to reduce the delay time for fast transient response.
Hysteretic control has very small delay time because it has no compensation network and the delay time is only determined by the performance of the comparator. Hysteretic control is carried out by comparing the feedback voltage signal and reference voltage with a hysteretic window. When output voltage is larger than the lower reference voltage, it outputs high level drive signal, and vice versa, low level or no drive signal is generated. The main disadvantage of hysteretic control is the variable switching frequency related with the hysteretic window, the variable switching frequency makes it difficult to design the converter filter. At the same time the parasitic resistance and inductance of printed circuit board traces, output capacitors and connectors strongly affect the functions of hysteretic control, and it is very sensitive to the noises. As a result, the hysteretic control is very difficult for high slew rate applications.
Active voltage positioning is another method used for fast transient response, which means the dc output voltage of the converter is dependent on the load current. The output voltage is set to the highest level at no-load condition and to the lowest level at full load. This approach increases the output voltage transient tolerance as much as twofold in the voltage regulator. Active voltage position can achieve almost dc constant output impedance of the converter. Active voltage position method is an improved linear control. In transient periods, the delay time of the compensation network still introduces voltage spikes and it is impossible to eliminate the delay time because of the requirement of the converter stability.
With the increasing demand for better dynamic performance under high slew rate output current, smaller delay times in a linear controller are required to improve the transient response. What is needed is a combination of linear and adaptive non-linear control to reduce the delay times for fast transient response, and it simplifies linear controller design.